Center-fed waveguide antenna



Dec. 31, 1957 i J. B. RANKIIQ 2,818,566

CENTER-FED WAVEGUIDE ANTENNA Filed Nov. 18, 1954 v 2 Sheets-Sheet 2INVENTORY JOHN B. RANKIN waveguide.

22,818,555 CENTER-'FEDYWAVEGUIDEANTENNA John Bruce Rankin, skillmaneNfiLa'ssi'gnor toRadio Corporation of America, a corporation of Delaware Theinvention relates to antennas. and particularly to ahighgain,omnidirectional, center-fed waveguide antenna capable ofhandlinghighpowerfor iiltra highfrequency broadcasting, and thefeedingsystem therefor.

Antenna arrays for transmitting ultrahigh frequency signalshave beendevised which consist of radiating elements equally spaced about thecircumference of a cylindrical waveguide. 'In the utilization of *suchan antenna array, .forexample, in television broadcasting, picture andsound energy travel-from one end of the antenna .array along the lengthof the antenna to diiferent radiating layers. One disadvantage of'suchan end fed antenna system is that the radiated beam will tiltvertically with frequency. A center-fed antenna maybe :used to removethis unwanted tilting of the transmitted signal.

In-a center-fed antenna used for television broadcasting, both thepicture and accompanyingjsound signal 'may'be diplexed into the sameantenna by a hollow pipe waveguide or a transmission line arrangementfrom'a part of the feed system for the antenna .withoutthe rnecessityfor separate frequency selective ,filters. Antennas for televisionbroadcasting which utilize coaxial and other wave transmissionlines inthis type of antenna '.feeding system are limited with respecttotheirpower handling capabilities. This invention utilizes pipe waveguides. of large dimensions throughout .its construction "because of theattendant higher power handling capabilities.

An object of the invention islto obtain an improved center-fed antennafor ultra highfrequency transmission utilizing pipe waveguides of largedimension'throug'hout.

Another object is to obtain'the centerfeeding of, signals from a singlehollowpi'pe waveguide a single surrounding coaxial waveguidewhichexc'ites a plurality of antenna elements.

Theinventionmakes useof a center-fed antenna system comprising arectangular waveguide 'into which or from which is fed an ultra-highfrequency signal. The rectangular waveguide 'is secured at'the free endto a circular,'hollowwave'guide by means .of a tapered circularto-rectangular coupling means or transition. .A section of a secondrectangular waveguide is positioned at right angles to the firstidentified rectangular waveguide and secured to the circular waveguide.An absorbing resistor serves to terminate the second identifiedrectangular waveguide. I V

The antenna array includes the above-mentioned circular, hollowwaveguide or "inner waveguide to which is connected at one end'bymeanso'f the tapered transition the first identified rectangularwaveguide, whereby energy maybe fed to or taken from .the circularwaveguide. A

.second circular or outer'waveguideis positioned so .that

substantially one-halfof its total surface area surrounds the-effectiveportion of the first mentioned circular, hollow A plurality of radiatingelements in-layers are equally spaced abouttlrecircumference.oftthesecond or outer circular waveguide. Coupling.Pl'ObfiS PDSiii-Ollfid adjacent to the end ofthe first mentionedcircular wave- 2,818,566 Ratented Dec. 31, 1957 "ice guide serve totransfer energy from the inner waveguide will be capable of handlinghigh power.

A more detailed description follows in conjunction with the accompanyingdrawingin which like reference numerals refer tolike parts throughoutthe figures of the drawing and in which:

Fig. 1 is a diagrammatical view of an omnidirectional antennalarray inaccordance with the invention and the feeding system therefor;

Fig. 2 is an enlarged elevation of the antenna array" 1 shown in Fig. 1;

Fig. 3 .is a view of a layer of radiating elements of the antenna arraytaken immediately below line 3-3 of Fig. 2;

Fig. 4 is a view of the layer of coupling probes between the waveguidesof the antenna array taken immediately "below line 44'of Fig. 2;

Fig. 5 is a perspective view of a section of the antenna feeding systemshown in Fig. 1 including the rectangular- .to-circu'lar waveguidetransition.

Fig. ,6 isa plan view of the section of the antenna feeding system shownin Fig. 5.

Referring to Figs. 1 and 2, an antenna array 1 which may be mounted as atower or fastened to a suitable support, not shown, includes a cylinder2. A metallic short- .circuiting plug 5 is positioned in the cylinder 2at a distance removed from one end thereof. A second cylinder 3 ofgreater diameter than that of cylinder 2 is mounted so as to surroundthe cylinder 2, approximately one-half of the total surface area of thecylinder 3 being located below the'short-circuiting plug 5. An antennaarray consisting of two pipe waveguides has been constructed. Theportion of cylinder 2 terminated by plug 5 constitutes one waveguide .9,the outer surface of cylinder 2 and cylinder 3 forming a second coaxialwaveguide 6. Coupling probes 4 are mounted so as to pass through thewall of the inner waveguide 9 in proximity to the short-circuiting plug5 and serve to couple energy traveling in the inner waveguide9 to thecenter of the outer waveguide 6. A number of radiator probes in the formof unipoles 10 are arranged in layers and are equally spacedcircumferentially about the surface of the array 1. The ends of .theouter waveguide 6 are terminated by short-circuitingiplugs 7 and 8, thepositions of which are adjustable. Energy traveling in the innerwaveguide 9 will be coupled to the center of the outer waveguide 6. Theenergy so coupled will then travel up and down the waveguide 6 to theshortclrcuiting plugs 7 and 8, respectively. The energy in the waveguide6 will be coupled from the waveguide to free space by the radiatorprobes 10 positioned about the surface of the array 1.

By making the short-circuiting plug 5 adjustable, it may be used for thematching of the antenna array by altering its position in the waveguide9 above coupling probes 4, as well as for preventing energy fromcontinuing on in the waveguide 9 beyond plug 5.

The arrangement of radiator probes 10 may be .more clearly understood byan examination of Fig. 3 showing a view of the probes 10 of the array 1taken immediately below line 33 of Fig. 2. Each layer of the arraycomprises eight probes 10 in the form of 'unipoles :equallyindividualbrass bushing 20 and insulatedfrom cylinder 3 by an insulator21 which may be made of a polytetrafiuorethylene material marketed underthe trade name Teflon.

The arrangement of coupling probes may be more clearly understood by anexamination of Fig. 4 which shows a view of the probes 4 of the array 1taken immediately below line 44 of Fig. 2. Eight probes 4 are equallyspaced circumferentially about the surface of the array 1, the endsthereof extending into waveguide 9 at substantially right angles to thelongitudinal axis thereof. The probes 4 are slidably secured to thecylinder 3 by brass bushings 23 and insulated from cylinder 2 byinsulator 22 which may be of the same material as used to insulate theradiator probes it) from cylinder 3. The energy traveling in hollowwaveguide 9 will be coupled to coaxial waveguide 6 by the properinsertion of the probes 4 into the waveguide 9. While the couplingprobes 4 are positioned between the ends of waveguide 9, the exactposition thereof in relation to that of the shortcircuiting plug 5 willbe determined in accordance with the tuning and matching requirements ofthe arrangement which are brought about in part by the adjustment ofplug 5. By providing a plurality of coupling probes, the power handledby each probe is a fraction of total power. Adequate means for couplingthe energy between waveguides thus is obtained and satisfactoryoperation of the arrangement is insured.

The antenna array disclosed, due to the use of large pipe waveguidesthroughout is able to transmit signals of high power and of high gain.It is particularly suitable when used in the transmission of ultra-highfrequency television broadcasting signals. Such an arrangement is shownin Fig. 1. A picture signal from a source, shown for purposes ofillustration as rectangle 24, is con nected to a bridge network by acoaxial conductor 26. A source of sound signals, shown for purposes ofillustration as rectangle 27, is connected to the bridge network 25 by acoaxial conductor 28.

The bridge network 25 utilized serves to combine the picture and soundsignals coupled thereto into a common output and many various typessuitable for use herein are well known in the art. For example, aiilterplexer may be used. The filterplexer performs the two functions ofcombining picture and sound and serving as the sideband filter for thepicture transmitter. The output of the bridge network 25 is arectangular waveguide 29.

At the base of the antenna array 1 is a network for connecting therectangular waveguide 29 and another rectangular waveguide 36 to thecommon circular wave guide 9. Energy from the rectangular waveguideswill be coupled into the circular waveguide 9 as, for example, linearlypolarized TE modes, the two modes oriented at 90 with respect to eachother. A commonly used network for this purpose includes a taperedtransition 31 from rectangular waveguide 29 to circular waveguide 9. Thesecond rectangular waveguide 30 is brought into the side of the circularwaveguide 9 so that its broad faces are parallel to the axis of thecircular waveguide 9 and perpendicular to the broad faces of the firstrectangular waveguide 29. The angular length of the tapered transition31 is chosen so that when a matched load is connected to the circularwaveguide 9, the rectangular waveguide 30 looking in toward the junctionof waveguides will be matched. A matched absorbing resistor 32 serves toterminate the rectangular waveguide 30. The circular, hollow waveguide 9becomes the inner waveguide of the antenna.

Referring to Figs. 5 and 6, showing views of a section of the antennafeeding system shown in Fig. 1, a circular polarizer is positionedbetween the tapered transition 31 and the central coupling probes 4 ofthe antenna array 1. A simple form of matched circular polarizerconsists of a pair of axial fins 33 and 3 attached to the inside of thecircular waveguide 9 and spaced one hundred and eighty degrees apart. Aplane passing through both fins should be inclined 45 with respect tothe surfaces of the rectangular waveguide 29. It is to be noted that thefins 33 and 34 have not been shown in Fig. 1 because they are not in theplane of the drawing. The fins will cause the TE mode containing pictureand sound signals to be circularly polarized. If a signal were fed fromrectangular waveguide 30, the fins would cause the resulting TE mode inthe circular waveguide 9 to be circularly polarized with oppositerotation.

The eight equally spaced probes 4 at the center of waveguide 6 andadjacent to the end of waveguide 9 transfer the energy from the innercircular, hollow waveguide 9 to the outer coaxial waveguide 6 formed bythe outer surface of the inner waveguide 9 and cylinder 3. In the innercircular waveguide 9, energy which is reflected by the probes 4 travelsback down the guide as a counterrotating circularly polarized TE mode.The fins reconvert the mode to linearly polarized TE mode oriented atwith respect to the original signal. This reflected energy then feedsinto the side rectangular waveguide 30 and is absorbed by the resistor32.

An antenna array and feeding system therefor has been disclosed forsuccessfully transmitting an ultra-high frequency television broadbandsignal of high power. Pipe waveguides are used as transmission linesthroughout. The picture and sound energy fed to the inner waveguide 9 asa circularly polarized TE mode is coupled to the outer waveguide 6 bythe probes 4. It is to be noted that no mode conversion will take placeat this location. The energy coupled into the outer waveguide 6 willtravel up and down the waveguide as a counter-rotating circularlyporalized TE mode. The energy will thereafter be coupled tofree space bythe radiator probes 10 which are tuned so as to be excited by the energytraveling in waveguide 6.

The proper tuning and matching of the antenna array may be accomplishedby altering mechanically the position of several of the componentsthereof. The location of short-circuiting plug 5 may be altered to allowthe proper coupling between the two waveguides 9 and 6 by couplingprobes 4. The coupling probes themselves are variable and may beslidably positioned to provide the proper tuning and matching. Theextent of insertion of the ends of probes 4 into waveguide 9 willdetermine the amount of energy abstracted from waveguide 9. The radiatorprobes 10 are also slidably adjustable and may be used to vary thetuning and matching of the array. Shortcircuiting plugs 7 and 8 areadjustably positioned at the ends of coaxial waveguide 6 to aid in theproper matching of the radiator probes. A further means of control is inthe selection of ratio of the cylinder diameters which is a variable andmay be used to control the coaxial guide impedance.

An actual embodiment of the invention constructed to operate on afrequency range in the area of 800 me. would have the followingelectrical and physical dimensions. The wavelength of the signal in freespace would be 14% (inches). It would be 24%," in the inner waveguide 9and 15.9 in the outer waveguide 6.

The outer diameter of the cylinder 3 would be 14%.", the inner diameterthereof being 14". The outer diameter of the cylinder 2 would be 11",the inner diameter thereof being 10%". The distance between thelongitudinal axis of each one of the coupling probes 4 and the bottom ofthe plug 5 would be or 18 measured in the waveguide 9. The distance'between the same longitudinal axis of the coupling probes 4 and thelongitudinal axis of the adjacent radiator probes 10 would be %)r or11.9 as measured in waveguide 6. The radiator probes 10 extending alongthe aperture of the antenna on each side of the coupling probes 4 wouldbe one wavelength in the outer waveguide 6 or 15.9" apart. Theshort-circuiting plugs 7' and 8 would be positioned M4 in the outerwaveguide 6 or '4" removed from the longitudinal axis of the endradiator probes 10. The length of the radiator probes will beapproximately M4 or 3 The length of the coupling probes 4 will be ofapproximately the same value but, as in the case of the radiator probes10, are slidably variable to allow for the proper tuning of the antennaarray.

What is claimed is:

1. An antenna comprising a hollow waveguide, a second waveguidesurrounding said first mentioned hollow waveguide, the outer surface ofsaid first hollow waveguide constituting the inner surface of the secondwaveguide, circumferentially positioned probes extending into the saidfirst mentioned waveguide at points removed from the end and atsubstantially right angles to the longitudinal axis thereof, said probesalso extending into the said second waveguide substantially at thecenter thereof, a short-circuiting plug extending across said firstwaveguide at a point intermediate the ends of said second waveguide forpreventing the flow of energy in said first waveguide therebeyond,energy traveling in said first waveguide being coupled to said secondwaveguide by said probes, and means for coupling said energy from saidsecond waveguide to free space.

2. An antenna comprising a hollow pipe waveguide, a cylinder, saidcylinder surrounding said hollow pipe waveguide, the outer surface ofsaid hollow pipe waveguide constituting the inner surface of a coaxialwaveguide in conjunction with said cylinder, circumferentiallypositioned probes extending into said hollow pipe waveguide at a pointremoved from the end and at substantially right angles to thelongitudinal axis thereof, said probes also extending into said coaxialwaveguide substantially at the center thereof, a short-circuiting plugextending across said hollow pipe waveguide intermediate the ends ofsaid coaxial waveguide for preventing the flow of energy in said hollowpipe waveguide therebeyond, and means for coupling energy transferredfrom said hollow pipe waveguide to said coaxial waveguide by said probesto free space.

3. An antenna comprising a hollow pipe waveguide, a cylinder, saidcylinder surrounding said hollow pipe waveguide, the outer surface ofsaid hollow pipe waveguide and said cylinder forming a coaxialwaveguide, short-circuiting plugs positioned at the open ends of saidcoaxial waveguide, a short-circuiting plug extending across said hollowpipe waveguide at a point intermediate the ends of said coaxialwaveguide for preventing the flow of energy in said hollow pipewaveguide therebeyond, coupling probes secured to said cylinder andextending into said hollow pipe waveguide at a point removed from theend and at right angles to the longitudinal axis thereof, said probesbeing equally spaced circumferentially about the center of said coaxialwaveguide, and means coexistent with said coaxial waveguide for couplingenergy coupled to said coaxial waveguide from said hollow pipe waveguideby said probes to free space.

4. An antenna comprising a hollow pipe waveguide, a cylinder, saidcylinder surrounding said hollow pipe waveguide, the outer surface ofsaid hollow pipe waveguide and said cylinder forming a coaxialwaveguide, a metallic short-circuiting plug mounted intermediate theends of said coaxial waveguide in said hollow pipe waveguide,short-circuiting plugs mounted at the open ends of said coaxialwaveguide, a layer of coupling probes secured to said cylinder wall andextending into said hollow pipe waveguide at a point removed from saidfirst mentioned short-circuiting plug and at right angles to thelongitudinal axis of said hollow pipe waveguide, said probes beingequally spaced circumferentially about the center of said coaxialwaveguide, a plurality of layers of radiator probes extending into freespace at right angles to the surface of said coaxial waveguide, saidradiator probes in each of said layers being equally spacedcircumferentially about said antenna whereby energy coupled be- 6 tweensaid waveguides by said coupling probes is coupled to free space.

5. An antenna comprising a hollow pipe waveguide, a cylinder, saidcylinder surrounding said hollow pipe waveguide, the outer surface ofsaid hollow pipe waveguide and said cylinder forming a coaxialwaveguide, a short-circuiting plug mounted intermediate the ends of saidcoaxial waveguide in said hollow pipe waveguide, short-circuiting plugsmounted at the open ends of said coaxial waveguide, a layer of couplingprobes secured to said cylinder wall and extending into said hollow pipewaveguide at a point removed from said first mentioned short-circuitingplug and at right angles to the longitudinal axis of said hollow pipewaveguide, said probes being equally spaced circumferentially about thecenter of said coaxial waveguide, a plurality of layers of radiatorprobes extending into free space at right angles to the surface of saidcylinder, said radiator probes being secured to the wall of said hollowpipe waveguide and passing through insulated apertures in the wall ofsaid cylinder, the radiator probes in each of said layers being equallyspaced circumferentially about said antenna whereby energy coupledbetween said waveguides by said coupling probes is coupled to freespace.

6. An antenna comprising a hollow pipe waveguide, a cylinder, saidcylinder surrounding said hollow pipe Waveguide, the outer surface ofsaid hollow pipe waveguide and said cylinder forming a coaxialwaveguide, a short-circuiting plug mounted intermediate the ends of saidcoaxial waveguide in said hollow pipe waveguide, short-circuiting plugsmounted at the open ends of said coaxial waveguide, a layer of couplingprobes adjacent said first mentioned short-circuiting plug secured tosaid cylinder wall and extending into said hollow pipe waveguide at apoint removed from the end and at right angles to the longitudinal axisof said hollow pipe waveguide, said probes being equally spacedcircumferentially about the center of said coaxial waveguide, means forpropagating energy of a circularly polarized TE mode in said hollow pipewaveguide, said energy being coupled to said coaxial waveguide by saidcoupling probes without mode conversion, a plurality of layers ofradiator probes extending into free space at right angles to the surfaceof said cylinder, said radiator probes being secured to the wall of saidhollow pipe waveguide and passing through insulated apertures in thewall of said cylinder, the radiator probes in each of said layers beingequally spaced circumferentially about said antenna whereby energycoupled between said waveguides by said coupling probes is coupled tofree space.

7. An antenna comprising a hollow waveguide, a second waveguidesurrounding said first waveguide so that substantially one-half of thetotal surface area of said second waveguide surrounds said firstwaveguide, the outer surface of said first waveguide constituting theinner surface of said second waveguide, circumferentially positionedprobes extending into said first waveguide at points removed from theend and at right angles to the longitudinal axis thereof, said probesalso extending into said second waveguide at the center thereof, ashort-circuiting plug extending across said first waveguide at a pointintermediate the ends of said second waveguide for preventing the flowof energy in said first waveguide therebeyond, energy traveling in saidfirst waveguide being coupled to said second waveguide by said probes,and means for radiating said energy from said second waveguide to freespace.

References Cited in the file of this patent UNITED STATES PATENTS2,408,435 Mason Oct. 1, 1946 2,658,143 Fiet et al. Nov. 3, 19532,705,305 Bailey Mar. 29, 1955

